This invention relates to electrical induction logging systems for determining the nature and characteristics of the various sub-surface formations penetrated by a borehole in the earth. More particularly, this invention relates to a high resolution array induction sonde that provides better vertical resolution while retaining the level of depth of penetration of prior tools.
It is important to the oil and gas industry to know the nature and characteristics of the various sub-surface formations penetrated by a borehole because the mere creation of a borehole (typically by drilling) usually does not provide sufficient information concerning the existence, depth location, quantity, etc., of oil and gas trapped in the formations. Various electrical techniques have been employed in the past to determine this information about the formations. One such technique commonly used is induction logging. Induction logging measures the resistivity (or its inverse, conductivity) of the formation by first inducing eddy currents to flow in the formations in response to an AC transmitter signal, and then measuring a phase component signal in a receiver signal generated by the presence of the eddy currents. Variations in the magnitude of the eddy currents in response to variations in formation conductivity as reflected as variations in the receiver signal. Thus in general, the magnitude of a phase component of the receiver signal, that component in-phase with the transmitter signal, is indicative of the conductivity of the formation.
U.S. Pat. Nos. 3,340,464; 3,147,429; 3,179,879; 3,056,917 and 4,472,684 are illustrative of typical prior-art well logging tools which utilize the basic principles of induction logging. In each of the tools disclosed in these patents, a signal generator operates to produce an AC transmitter signal which is applied to a transmitter coil. The current in the transmitter coil induces in the formations a magnetic field, which, in turn, causes eddy currents to flow in the formations. Because of the presence of these formation currents, the magnetic field of the transmitter is coupled into a receiver coil R thereby generating a receiver signal. (Logging tools having "a receiver coil" and "a transmitter coil" each comprising several coils arranged in a predetermined geometrical fashion to obtain a desired response are commonly used. These coil systems are sometimes referred to as "focused" coil systems.) The receiver signal is then generally amplified and applied to one or more phase sensitive detectors (PSDs). Each PSD detects a phase component signal having the same phase as a phase reference signal which is also applied to the detector. The phase reference signal has a predetermined phase relationship to the current in the transmitter coil(s). The output of the PSD(s) may be further processed downhole, or may be sent uphole to surface equipment for processing or display to an operating engineer. Such processing may be accomplished using many well known techniques, including phasor deconvolution processing taught by U.S. Pat. Nos. 4,513,376 issued to T. Barber on Apr. 23, 1985, and 4,471,436 issued to R. Schaefer and T. Barber on Sept. 11, 1984, both incorporated by reference.
Since the earliest patents pertaining to focussed coil systems for induction logging (U.S. Pat. Nos. 2,582,313 and 2,582,315) the art has attempted to reduce the contribution to the tool response made by that part of the formation invaded by the drilling fluid ("invaded zone") and by formations above and below the region of interest. For example, U.S. Pat. No. 3,067,383, issued to D. R. Tanguy on Dec. 4, 1962 and incorporated herein by reference, discloses a sonde (hereinafter referred to as the Tanguy sonde) that has been very widely used in the industry and U.S. Pat. No. 2,790,138 issued to A. Poupon on Apr. 23, 1957 discloses the use of a plurality of electrically independent transmitter-receiver pairs arranged symmetrically about the same center point. The response of that tool is obtained by combining the response of the several electrically independent pairs, these pairs being arranged in such a manner that contributions to the tool response from formation regions lying above or below the outermost coils and from the formation region close to the borehole are reduced. These attempts to increase vertical resolution and achieve greater depth of formation penetration have not been entirely successful.
Other patents (such as U.S. Pat. No. 3,329,889 issued to D. R. Tanguy on July 4, 1967 and U.S. Pat. No. 3,453,530 issued to G. Attali on July 1, 1969), incorporated by reference, has described induction tools that combine two or more focused arrays in one tool with the object of making measurements in radially different parts of the formation. The deep array in these tools is the deep-reading Tanguy sonde, with a derivative array having several receivers and sharing common transmitters with the deep array forming an array with a medium depth of investigation. The deep and medium arrays from these references will be referred to as ILD and ILM, respectively.
Until recently, strong deconvolution methods for induction sondes have failed in practice, because the response of an induction sonde is a nonlinear function of the formation conductivity. Any deconvolution method that does not correct for this nonlinearity will fail when applied to real log data. On the other hand, attempts to produce a strongly focussed induction array have also had problems. Unlike radar antennas, an induction sonde can be "focused" in the array only by adding coil pairs that subtract out signal from where it is not wanted, leading to very low sensitivity. Another problem with strong focussing is the length necessary to produce a narrow response lobe.
With the advent of the phasor deconvolution processing methods, noted previously, with its skin effect correction, deconvolving the nonlinear induction response function became possible. The application of phasor deconvolution to prior tools has produced deep-reading logs free of shoulder effect, but with no increase in vertical resolution. The resolution problems lie therefore, in the response function of the deep-reading tools themselves. With the present state of the art in signal processing, the only solution to these problems is to use shallower arrays whose response function contains sufficient high spatial frequency information as an addition to the deep measurement. The combination can be successfully sharpened with a deconvolution filter. The present invention provides such a combination of induction array tools and signal processing.
It is therefore an object of the invention to provide a novel method of combining measurements from shallow and deep induction arrays to produce an equivalent array capable of better vertical resolution and greater radial depth of penetration than prior tools.
A further object is to provide a novel dual induction or multiple induction tool having complementary arrays that provide various radial depths of penetration.
Other and further objects will be explained hereafter and are more particularly delineated in the appended claims.
In summary, however, from one of its broad aspects, the invention contemplates a method and apparatus for enhancing the resolution of an induction logging tool at a desired depth of investigation by transmitting an induction signal into the formation, receiving at a first receiver (array) a first induction signal from the formation corresponding to the desired depth of investigation, receiving at a second receiver (array) a second induction signal from the formation having no zeroes in the spatial frequency response in the frequency band of interest and combining the first and second received induction signals to provide a high resolution measurement of a characteristic of the formation at the desired depth of investigation. Preferred details and structures are hereinafter more particularly described.